The Huexotzinco Codex, 1531, documenting in pictographic language part of the testimony in a legal case against representatives of the colonial government in Mexico, ten years after the Spanish conquest in 1521. Photograph: Library of Congress

Libraries and archives from around the world have come together in a project to share their collections of rare books, maps, films, manuscripts and recordings online for free.

Almost four years in the making, the World Digital Library will launch on 21 April, functioning in seven languages – Arabic, Chinese, English, French, Portuguese, Russian and Spanish – and including content in additional languages. A prototype of what will be on offer includes a voice recording of the 101-year-old grandson of an American slave, a 17th-century map of the world and 19th-century Brazilian photographs.

The brainchild of James Billington, from the US's Library of Congress, the project has been developed by Unesco and the Library of Congress, along with 32 other partners from around the world, including national libraries from Iraq, Egypt, Russia, Brazil, Israel, Saudi Arabia and Uganda.

As well as documents and recordings, the digital archive will also feature videos from curators explaining why they are important and what they reveal about a culture, and the option to translate material into different languages. Users will be able to browse and search by place, time, topic and type of item, with material to also include musical scores, prints, photographs, architectural drawings and other significant cultural materials.

The World Digital Library says its objectives are to promote international understanding, to expand non-English and non-western content online, as well as to contribute to research and education. Billington, when he proposed the idea in June 2005, said it would "hold out the promise of bringing people closer together precisely by celebrating the depth and uniqueness of different cultures in a single global undertaking".

Late last year the EU unveiled the prototype of Europeana, an online archive of European culture to which more than 1,000 European national libraries, museums and institutions have contributed content. It proved so popular on its launch in November, with 10m hits an hour, that it had to be temporarily closed.